A. Field of the Invention
The present disclosure relates to the field of aerogels. In particular, the invention concerns aerogels made from a branched polyimide matrix having low, or substantially no crosslinked polymers.
B. Description of Related Art
An aerogel is a porous solid that is formed from a gel, in which the liquid that fills the pores of the solid has been replaced with a gas. Shrinkage of the gel's solid network during drying is negligible or all together prevented due to the minimization of or resistance to the capillary forces acting on the network as the liquid is expended. Aerogels are generally characterized as having high porosity (up to about 94-98%), and high specific surface area. Aerogels also possess relatively low densities and are unique solids with up to 99% porosity. Such large porosities confer a number of useful properties to aerogels, including high surface area, low refractive index, low dielectric constant, low thermal-loss coefficient, and low sound velocity.
Aerogels made from organic polymers (e.g., polyimides or silica/polyimide blends) provide lightweight, low-density structures; however, they tend to exhibit low glass transition temperatures and degrade at temperatures less than 150° C. Attempts to improve the thermal properties of the aerogels have included cross-linking tri, tetra, or poly-functional units in the structure. NASA Technical Brief LEW 18486-1 describes polyimide aerogels having three-dimensional cross-linked tri-functional aromatic or aliphatic amine groups or, in the alternative, capping long-chain oligomers with latent reactive end caps that can be cross-linked after a post cure of the dried gels. U.S. Pat. No. 8,974,903 to Meader et al. discloses a porous cross-linked polyimide-urea network that includes a subunit having two anhydride end-capped polyamic acid oligomers in direct connection via a urea linkage. U.S. Pat. No. 9,109,088 to Meader et al. discloses cross-linked polyimide aerogels that include cross-linked anhydride end-capped polyamic acid oligomers. While these cross-linked polyimide aerogels have demonstrated good mechanical properties, they are difficult to manufacture commercially, and cross-linked polymers are difficult to reprocess or recycle. The lack of manufacturability and recyclability can have a negative impact on production scale-up, large-scale manufacturing, conformation to irregular surfaces, or maintaining integrity in dynamic conditions.